At the U.S. border fence with Mexico, just outside of Nogales, Arizona, handprints are left on the iron posts when people try and scale the wall to get into the U.S. These prints are everywhere, and they go right to the top. Border patrol agents said they can often identify people who have just illegally climbed the fence by the telltale rust stains on their hands and clothing.

This is part of an ongoing reporting trip for the PBS NewsHour about the border, including the challenges of...